Personal computers, workstations, and servers are general-purpose devices that can be programmed to automatically carry out arithmetic or logical operations. These devices include at least one processor, such as a central processing unit (CPU), and some form of memory system. The processor executes instructions and manipulates data stored in the memory.
Memory systems commonly include a memory controller that communicates with some number of memory modules via multi-wire physical connections called “channels.” Each memory module commonly includes dynamic random access memory (DRAM) components mounted on a printed circuit board. Successive generations of DRAM components have benefited from steadily shrinking lithographic feature sizes. Storage capacity and signaling rates have improved as a result.
One metric of memory-system design that has not shown comparable improvement is the number of modules one can connect to a single channel. Adding a module to a channel increases the “load” on that channel, and thus degrades signaling integrity and limits signaling rates. The number of modules per memory channel has thus eroded with increased signaling rates. There is therefore a demand for increased module counts per channel.
Reliability is another important metric of memory-system design, and can be characterized in terms of mean time between failures. Adding complexity, such as additional modules per channel, increases the number of elements that might fail. Reliability can be improved by careful engineering, and modern computer systems are a testament to that care. Hardware faults remain a possibility, however. The most reliable systems detect and manage these and other problems. There nevertheless remains a need for ever more reliable systems.